Renal micropuncture techniques and microperfusion of renal tubules in vivo will be utilized to measure pressures, flows, permeabilities and protein concentration for the proposed work. Studies will include examining the relationship between glomerulo-tubular balance and tubulo-glomerular feedback. Changes in nephron filtration rate will be measured with alterations in distal perfusion rates to quantify absolute and fractional proximal reabsorption. These studies will include examination of tubulo-glomerular feedback in animals at filtration pressure disequilibrium, but at normal blood flow rates. These studies will involve the measurement of glomerular determinants of filtration and perfusion of distal tubules in rats with a positive effective filtration pressure in the glomerulus without change in volume status of the rat. Additionally, we plan to study the influence of hydrostatic and colloid osmotic pressure on fluid reabsorption in the descending limb of the Loop of henle. This will be done in an isolated tubule perfusion system utilizing nephron segments from rabbit kidneys.